Chemistry, more like cheMYSTERY to me! – Stoichiometry

At this point in the year, the curriculum is getting more difficult and is building to what I call “the top of chemistry mountain.” I call stoichiometry the top of chemistry mountain because it pulls together the big picture of chemistry: chemical reactions, balanced equations, conservation of mass, moles and even gas laws! One of my students depicted the harrowing climb below:

Let’s recap the climb from Unit 7 before we jump in:

Molar masses on the periodic table are relative to 12 g of Carbon-12 or 1 mole of carbon

There are 6.02 x 10^23 particles in a mole

Empirical formulas represent the simplest ratio in which elements combine and can be calculated using mole ratios

Molecular formulas represent the actual number of atoms of each element that occur in the smallest unit of a molecule. This may be the same as the empirical formula.

This unit is long so you might want to pack a snack!

I start Unit 8 with an activity my students always beg me for from the first time they use Bunsen burners: making s’mores. Of course, those s’mores cost them some chemistry!

S’mores Stoichiometry

S’more stoichiometry is a fun and easy activity to introduce students to the idea of reaction ratios and even limiting reactants. A s’more can be made with the balanced equation:

Gm2 + 2Ch + Mm –> Gm2Ch2Mm

Where Gm is the diatomic element graham cracker, Ch is chocolate and Mm is marshmallow. Students go through a series of calculations converting between mass of ingredients and number of ingredients (mass of reactant to moles of reactant) and then to quantity of s’mores (moles of reactant to moles of product). Students even complete a limiting reactant problem when given a finite amount of each ingredient. The reward for all this math? Delicious, gooey, Bunsen burner s’mores.

Now that they have gotten the marshmallow roasting out of their systems, it is time to start the final ascent to the top of chemistry mountain!

BCA Tables

I love a lot of things about the Modeling Instruction curriculum, but BCA tables might be my favorite. If you are not familiar with BCA tables, check out the ChemEdX article I wrote here. BCA tables are an awesome way to help students think proportionally through stoichiometry problems instead of memorizing the mass-moles-moles-mass algorithm. I introduce BCA tables giving students moles of reactant or product. I add mass, percent yield, molarity, and gas volumes one by one as “add-ons” to the model.

Percent Yield Lab

The first “add-ons” are theoretical yield and percent yield. Students react solutions of sodium carbonate and calcium chloride (mass and mixed by students) to form calcium carbonate. Students gravity filter (I do not have aspirators in my room for vacuum filtration) the precipitate and dry it. While waiting for the product to dry, students calculate their theoretical yields. This calculation requires students to realize they need to convert their masses of reactants to moles before using a BCA table and then convert the moles of product from the BCA table to mass of product. After drying, students are able to calculate their percent yields and discuss why this is an important calculation and what their possible sources of error are.

Molarity

The next “add-on” to the BCA table is molarity. This can be saved for after limiting reactant, depending on how your schedule works out. Students learned about molarity back in Unit 7 but it never hurts to review before you jump into the stoichiometry. Again, the key to keeping this simple for students is molarity is only an add-on. Only moles can go in the BCA table so calculations with molarity should be done before or after the BCA table.

Limiting Reactant PhET

Now that students are stoichiometry pros when given excess of one reactant, it is time to “adjust to reality” as the Modeling curriculum says. This year, I introduced the concept of limiting reactants with the “Reactants, Products and Leftovers” PhET. Students started by making sandwiches with a BCA table and then moved on to real reactions. This activity helped students visualize what it looks like to have left over product. The key to using the PhET is to connect every example to the BCA table model. Before switching from sandwiches to actual reactions, I have a quick whiteboard meeting to introduce the term “limiting reactant.”

Limiting Reactant Practice

After the PhET, students work on the “Adjusting to Reality” worksheet from the Modeling Instruction curriculum. This worksheet starts by giving students reactant quantities in moles and then graduates them to mass values. The BCA table helps students easily pick out the limiting reactant and helps them see how much reactant is leftover and how much product is produced in one organized table.

I then have students work on a worksheet I call “All the Stoichiometry” because it has all types of problems with all levels of difficulty to make sure students can discern when to use the different tools they have collected.

Chemistry Feelings Circle

When I have a really challenging problem that I think would take too long for individual groups to solve, I hold a chemistry feelings circle. I arrange all of my seats in a tight circle and place a pile of whiteboards and markers in the middle.

Every student must sit in the circle and the class must solve the problem together by the end of the class period. I act like I am working on something else but really I am taking notes about their conversations. Once all students have signed off on the solution, they can elect delegates to present it to me. This year, I gave students a zombie apocalypse challenge problem involving the 2-step synthesis of putrescine. Students had to determine whether they could synthesize enough putrescine to disguise all of their classmates. Spoiler alert, there is not enough!

Ideal Gas Law

With limiting reactant under our their belts, it is time for another stoichiometry add-on, the last one. It is time for the ideal gas law. I return to gas laws through the molar volume of a gas lab. Students know how to convert mass and volume of solution to moles. What about gas volume (I may bump this back to the mole unit next year)? That question leads to the challenge of determining the volume of 1 mole of gas at STP. I usually use the traditional gas collection over water set-up but this year I was gifted a class set of LabQuest 2’s and I wanted to try them out. I used the Vernier “Molar Volume of a Gas” lab set-up instead.

I am not sold on this procedure but it got us the data we needed. With the molar volume of gas at a STP, we can derive PV=nRT and calculate R (the universal gas constant).

Grab-bag Stoichiometry

At the top of chemistry mountain, I give students a grab bag of stoichiometry problems. They may have to convert reactant or product mass, solution volume/molarity or gas volume to/from moles in addition to completing a BCA table. I give students a flow chart to fill in to help them sort out the process.

Unit 8 Practicum

Once students reach the top of chemistry mountain, it is time for a practicum. I use Flinn’s micro-mole rocket activity for the practicum but I leave it very open ended. I show students that hydrogen gas reacts with oxygen gas to form water and this creates enough energy to power the rocket (pipet bulb). From there, I set them loose to figure out what volume of each gas they need and where to mark their rocket so they can fill the gas volumes correctly. I also have students do some fun (not the word my students might use to describe them) stoichiometry calculations (see below).

Stoichiometry Coding Challenge

I usually end a unit with the practicum but I really wanted to work a computer coding challenge into this unit. Asking students to generalize the math they have been doing for weeks proves to be a very difficult but rewarding task.

For the coding challenge, I ask students to write a series of cumulative programs in Python that build to a stoichiometry calculator. First, students write a simple code that converts between mass and moles.

Then they write similar codes that convert between solution volume and moles and gas volume and moles. Students then combine those codes to create a calculator that converts any unit to moles. Once students have the front end of the stoichiometry calculator, they can add in coefficients. Finally, students build the back-end of the calculator, theoretical yield. You can read my ChemEdX blog post here.

By the end of this unit, students are about ready to jump off chemistry mountain! Luckily, the rest of the year is a downhill ski.

Let’s see what we added to the model so far…

The coefficients in a balanced equation represent the molar ratios in which elements and compounds react

The theoretical yield for a reaction can be calculated using the reaction ratios

The percent yield for a reaction is based on the quantity of product actually produced compared to the quantity of product that should theoretically be produced.

The reactant that runs out first is called the limiting reactant because it determines how much product can be produced

The pressure, volume, temperature and moles of an ideal gas can be related through the universal gas constant